A control arrangement of a continuously variable power split transmission driven by an internal combustion engine is known (Steuerungen, August 1987, pages 38-36), Wherein a pedal gives a pedal path signal which together with an engine speed signal of the internal combustion engine is involved in the setting of the throttle valve opening and in the control of a servo adjusting device for the continuous variation of the power split transmission. The power split transmission, with a mechanical branch and a hydrostatic branch, comprises multishaft linkages, which superimpose the continuously variable engine speed (rpm) of the hydrostatic branch on the constant input engine speed of the mechanical branch. The mechanical branch of the power split transmission has several modes. The shifting of the modes in the mechanical branch takes place at a synchronous rotational speed of the output linkage shafts of the linkage. In the hydrostatic branch and the linkage input shaft directly driven by the internal combustion engine, the synchronous speed depends on the engine speed of the internal combustion engine The shifting of the stages is monitored by the control device, which, however, does not determine the frequency of these shifting processes.
An essential advantage of continuously variable drive units is that the internal combustion engine can be operated with constant engine speed over the entire speed range, for instance with an engine speed at which the internal combustion engine registers its best fuel consumption value. The changes in the vehicle speed are produced by changes in the transmission ratio. In this way, a well-defined relationship between the transmission ratio and the travel speed of the vehicle results. If the driver selects a speed for the long-term operation of the vehicle , this can be reached exactly at a transmission ratio which corresponds to the one at the shift point or close to one shift point of the mechanical branch of the transmission, so that, when the cruising speed variations are small, this shifting process between two neighboring speed ranges repeats itself frequently. Particularly for agricultural vehicles, which are used on very uneven terrain, the road resistance changes very often and suddenly, and the consequences are oscillations in the travel speed of the vehicle. If, for instance during work in the fields, the agricultural vehicle is operated with a preselected long-term speed coming closely to the one of the shift points, the oscillations in the travel speed lead to frequently repeated shifts between two neighboring speed ranges. This contributes to the increased wear and tear of the transmission in general, but especially of the clutches between linkages and transmission output shafts. Also, the efficiency of the transmission and the general comfort of the vehicle decrease.